1. Field of the Invention
The present invention relates generally to the structure of radially expansible lumenal prostheses, including stents and grafts. More particularly, the present invention relates to the provision of articulation structures for the construction of flexible and pseudo-flexible prostheses and the provision of end structures for the construction of atraumatically deliverable prostheses.
Lumenal prostheses are provided for a variety of medical purposes. For example, lumenal stents can be placed in various body lumens, such as blood vessels, the ureter, the urethra, biliary tract, and gastrointestinal tract, for maintaining patency. Lumenal stents are particularly useful for placement in pre-dilated atherosclerotic sites in blood vessels. Lumenal grafts can be placed in blood vessels to provide support in diseased regions, such as aortic abdominal, and other aneurysms.
Both stent and graft prostheses must meet certain mechanical criteria to function successfully. In particular, such prostheses should be at least partly flexible or articulated (such as rigid sections that articulate relative to one another) over their lengths so that they may be advanced through tortuous body lumens, such as those of the coronary vasculature. In addition, the prostheses should preferably maintain their original length or foreshorten minimally when the prostheses assume an expanded configuration. Further such prostheses must have sufficient mechanical strength, particularly hoop strength, in order to mechanically augment the lumenal wall strength and thus assure lumen patency. The ability to meet these requirements is severely limited in the case of cylindrical endolumenal prostheses which are delivered in a radially constrained or collapsed configuration. Such prostheses must radially expand at a target site within the body lumen, so any adaptations which are intended to enhance flexibility will not interfere with the ability to radially expand or to maintain strength once expanded.
Prior lumenal prostheses often have structures which present a risk of injury as they are endolumenally delivered (i.e. tracked) to and/or released at a target site within a patient""s body lumen. In particular, many vascular stents comprise a plurality of circumferentially connected and spaced-apart longitudinal elements which deform circumferentially as the stent is radially expanded. The Palmaz stent described in U.S. Pat. Nos. 5,102,417 and 4,776,337, is typical of such stents. When these prostheses are flexed or articulated during delivery or tracking, these longitudinal elements of conventional prosthesis tend to create a phenomenon known as xe2x80x9cfishscalingxe2x80x9d which occurs when these elements distributed along the length of the prosthesis protrude outward from the surface of the prosthesis. Such stents exhibit poor xe2x80x9ctrackingxe2x80x9d characteristics, where xe2x80x9ctrackingxe2x80x9d is defined as the ability to pass smoothly through tortuous pathways. These protruding elements along the length of the prosthesis increase the likelihood that the prosthesis will dig into or otherwise engage the wall of the body lumen during delivery and even arrest the progress of the prosthesis and its delivery system to the diseased region (or target site). Additionally, a lesser-known phenomenon called xe2x80x9cflaringxe2x80x9d occurs when the longitudinal elements of the distal or proximal end of the prosthesis are bent outward to assume a crown-like configuration due to bending forces placed on these elements as the prosthesis passes through tortuous body passageways. Flaring can create the same deleterious effects as the previously described fishscaling phenomenon, injuring or traumatizing the blood vessel wall as the prosthesis is delivered or tracked within the blood vessel.
A separate problem in stent construction and deployment relates to the ability to detect the stent fluoroscopically during the deployment procedure. Stainless steel, the most common stent material, is generally radiolucent, i.e. it is minimally visible under x-rays and permits fluoroscopic examination therethrough. Advantageously, such stents do not interfere with subsequent fluoroscopic examination of the treated region of the body lumen, such as six-month-followup examinations. They are, however, much more difficult to accurately position within the lumen due to their radiolucency. To increase radiopacity, these prostheses may be manufactured from radiopaque materials such as tantalum, platinum, or nickel titanium (NiTi). Alternatively, the entire prosthesis may be plated or coated with a uniform layer of radiopaque material to improve prosthesis visualization as disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 08/691,661, filed Aug. 2, 1996 , the complete disclosure of which is incorporated herein by reference. Although these methods address the issue of radiopacity, uniform layers of such materials or prostheses made entirely of such materials typically improve radiopacity at the cost of reduced visibility of tissue inside the prosthesis.
For these reasons, it would be desirable to provide improved stents and other lumenal prostheses. In particular, it would be desirable to provide improved lumenal prostheses and methods for their endolumenal placement, where the prostheses can be delivered or tracked to a target site within a body lumen without traumatically engaging the walls of the body lumen. Preferably, the prostheses will have elements which minimize xe2x80x9cfishscalingxe2x80x9d with its concomitant risk of injury or even retention within the body lumen. Such prostheses may also be provided with minimally traumatic end rings in order to reduce the risk of lumenal injury as the prostheses is both delivered and radially expanded within the body lumen. Optionally, the ends of the prostheses may incorporate stiffening elements which reduce the occurrence of prosthesis flaring or trumpeting during tracking through tortuous body lumens. Additionally, the prostheses will be radially expansible at the target location, and will preferably retain both their cylindrical configuration and flexibility or ability to articulate after expansion. Such prostheses should further have sufficient hoop strength and other mechanical characteristics so that they may effectively function as stents in maintaining lumenal patency and/or grafts in enhancing lumenal wall strength. To more precisely direct scaffolding force at a diseased site, the prosthesis may have specific expanded configurations. Furthermore, it would be desirable to provide improved prostheses having radiopacity characteristics which permit visualization of the prosthesis both during tracking and deployment as well as visualization of tissue within the lumen during subsequent angiographic followup after deployment. The present invention will provide at least some of the desired improvements.
2. Description of the Background Art
Vascular stents comprising multiple segments joined by axial hinge structures are described in U.S. Pat. Nos. 5,195,984; 5,104,404; and 5,102,417 and European Patent Publication EP 540 290. Other stent structures are described in U.S. Pat. No. 5,282,824, European Patent Publication EP 481 365; and Canadian Patent Publication 2,079,944. U.S. Pat. No. 4,776,337 describes the Palmaz stent which consists of multiple longitudinal box elements joined to each other by short circumferentially oriented tabs and usually having at least two such sections joined longitudinally by a single short beam as shown in U.S. Pat. No. 5,195,984.
The present invention provides improved prostheses and methods for their endolumenal placement within body lumens, particularly blood vessels. The prostheses may be in the form of stents, intended for maintaining lumenal patency, or may be in the form of grafts, intended for protecting or enhancing the strength of the lumenal wall. The prostheses of the present invention will be radially expansible, either by the application of an internal force to expand a minimally resilient (usually malleable) prosthesis structure or by release of radial constraint from a resilient prosthesis structure (self-expanding).
In a first aspect of the present invention, the prosthesis comprises a plurality of radially expansible unit segments, such as rings, having a plurality of beams connecting axially remote points on adjacent unit segments, and a plurality of compliant elements referred to hereinafter as expansion joints connecting axially proximate points on adjacent unit segments. The beams are usually aligned axially and maintain the remote points at a fixed distance at all times including when the prosthesis is radially expanded. The expansion joints, in contrast, allow for relative movement of the proximate points during bending, expansion, or any other deformation of the prosthesis. The expansion joint may have any convenient geometry, such as an xe2x80x9cSxe2x80x9d shape, a xe2x80x9cZxe2x80x9d shape, a serpentine pattern, a zig-zag pattern, or the like. Thus, the expansion joints will provide a compliant, flexible connection of the proximate points on the unit segments to reduce or prevent protrusion or xe2x80x9cfishscalingxe2x80x9d of these points as the prosthesis is delivered through a tortuous path or deployed (expanded) at a curved target site.
To create the tracking and expansion characteristics desired in the prosthesis, the beams and expansion joints may be positioned in particular patterns between each unit segment. For example, although adjacent unit segments of the prosthesis may be coupled by both expansion joints and beams, they are preferably joined by only one beam and some number of expansion joints. Having only one beam balances the benefits of length compensation and prosthesis flexibility. Beams of the prosthesis are preferably positioned in a xe2x80x9cladderedxe2x80x9d configuration where the sequence of longitudinal beams is circumferentially and longitudinally staggered, typically with a predetermined amount of longitudinal overlap between the beams when in the unexpanded configuration. The amount of overlap may be about one-third the length of each beam or more preferably one-half the length of each beam. In certain embodiments, beams may also have a weakened midportion of lesser width or lesser thickness than the remainder of the beam to increase transverse flexibility of the prosthesis. The expansion joints may also be weakened to maximize flexibility and minimize interference with the ability of the prosthesis to track. Typically, weakened expansion joints have smaller width or smaller thickness than the longitudinal struts in the unit segments.
In a second aspect of the present invention, the prosthesis may include elements to increase radial stiffness both when unexpanded and when expanded, such as an end segment requiring a greater radial expansion force than another segment in the middle of the prosthesis. In one embodiment, the end segment is a serpentine ring having longitudinal struts of shorter length than the longitudinal struts of segments in the middle of the cylindrical frame. Alternatively, the end rings could have increased stiffness by using hinges of increased width or thickness between each longitudinal portion or strut, as disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 08/691,661, filed Aug. 2, 1996, the complete disclosure of which has been previously incorporated herein. Increased radial stiffness at the ends of the prosthesis, such as through struts of shorter lengths or box structures with unequal strut lengths, tends to reduce flaring or trumpeting of these ends which may occur during tracking through tortuous passageways. A still further improvement for reducing prosthesis flaring or trumpeting involves using a radially stiffened end segment that is hinged by a long beam to an adjacent segment. The single beam provides articulation between the segments and thus reduces the load placed on the struts in the end segments. Additionally, a prosthesis can be mounted on an inflation member such as a balloon which can shape the expanded prosthesis into a noncylindrical shape. For example, the prosthesis may be expanded into a tapered configuration to provide improved fit and scaffolding in body lumens which have conical configurations. The ability of the inflation member, such as a noncylindrical balloon, to expand a prosthesis into a shape that better conforms to the lumenal geometry may also be enhanced by having unit segments of various radial stiffnesses, such as having a stiffness gradient from one segment to the next, or other nonuniform stiffnesses along the length of the prosthesis.
In a third aspect, a system according to the present invention includes a cylindrical frame forming a prosthesis having a plurality of radially expansible unit segments, a longitudinal length, a distal end and a proximal end. The frame is preferably mounted over an expansible cylindrical section of an inflation member such as a balloon. In this third aspect, the expansible cylindrical section has the same length as the length of the cylindrical frame. Advantageously, this minimizes expansion of the balloon beyond the ends of the prosthesis which may do dissectogenic damage, particularly to the distal the body lumen in which the system is delivered. The inflation section of the balloon is preferably no more than about 1 mm longer from each end of the typically cylindrical frame, more preferably about the same length as the cylindrical frame. The present invention also provides a kit with instructions for use describing a method for mounting a prosthesis over such a matching length balloon together with a mounting tool.
In a fourth aspect, a prosthesis according to the present invention may have a coating of radiopaque material such as gold covering at least the entire outer surface of the prosthesis. The coating will preferably have areas of a first desired thickness and other areas of a second desired thickness. In one embodiment, the ends of the prosthesis have a coating of 0.0006 inches of gold while the remainder of the prosthesis, typically the middle area, has a coating of 0.0003 inches. In another embodiment, a prosthesis may be created using the method of the present invention to create ends having a coating of 0.0006 inches of gold while the middle area has no coating at all. Advantageously, a prosthesis having different thicknesses of radiopaque material added to a radiolucent prosthesis will facilitate the accurate positioning of the prosthesis in the lumen while also allowing for tissue examination following the expansion of the prosthesis and during angiographic followup.
In a further aspect, the present invention provides a method for reinforcing the wall of a body lumen by introducing a prosthesis, having the proximal points of the unit segments constrained by expansion joints and the remote points of adjacent units segments joined preferably by a single beam, to a target site within the body lumen and radially expanding the prosthesis once the target site is reached. The prosthesis may be expanded by applying an expansive force, such as that supplied by a balloon catheter, or alternatively, the prosthesis may be released from a radial constraint at the target site. Alternatively, the method of the present invention may comprise delivering and then expanding a prosthesis having radially constrained or limited ends to reduce the likelihood of prosthesis flaring or trumpeting during delivery or tracking through tortuous body passageways.